Thousands of miles of buried natural gas pipes of varying size and formed from various materials are presently in service. All of these pipes are in some state of progressive degradation. In most instances, the extent of such degradation is unknown, and hence, the serviceability of the pipes is similarly unknown. This lack of information with the respect to the degree of degradation results in unforeseen gas pipe leaks and/or breaks, and necessitates the expending of substantial time and expense in locating these defects so that repairs and/or replacement can be made. Because of the need to detect conditions which might result in gas pipe breaks and/or leaks, an apparatus has been developed for inspecting gas pipes, and such apparatus is usually referred to as a pipe line "pig." Such pipe line "pigs" typically include a housing with a plurality of sensors, such as ultrasonic transducers, mounted to the outer surface thereof in a predetermined configuration or array to contact the inner surface of the gas pipe. Since the sensors are in a predetermined configuration or army, complete inspection of the wall of the gas pipe is generally not possible, i.e., the portion of the wall between two adjacent sensors is typically not inspected. Thus, to approach complete inspection of the wall would require an inordinate number of sensors. In an attempt to achieve close to complete inspection, several "pigs" have incorporated rotational movement of the sensors as the pigs advance axially down the pipe, thus producing a helical scan pattern. By increasing the rotational speed of the sensors, the helical pattern compresses, essentially increasing the percentage of the pipe inspected. A more thorough discussion of these principles is found in U.S. patent application Ser. No. 08/222,621, filed on Apr. 5, 1994, and entitled "Scan Assembly Structure", the disclosure of which is incorporated herein by reference.
However, because the pipe walls contain imperfections and are often littered with debris, ultrasonic energy introduced into a pipe wall from a transducer, at an angle perpendicular to the inside surface of the pipe at the point of entry, is not received back at the point of entry, but is reflected and refracted off the debris and/or the imperfections of the pipe at oblique or random angles. Thus, the transmitted energy is sufficiently scattered such that no response signal is detected. The absence of scan data at a particular place within a scan due to this phenomenon is referred to in the art as a "drop-out." Because of the nature of "drop-outs", merely increasing the mean rotational speed of the sensors during the scan of a discrete section of pipe does not improve the results. The debris and/or imperfection which cause a "drop-out" at one scan rate produce a similar "drop-out" at a different scan rate. Further, it is wholly impractical to make multiple scan passes of a discrete section of pipe in order to synthesize the data into a single tomographic representation of the pipe because of the extreme exactitude required to scan the exact same section multiple times. In other words, the sensors would have to be positioned in precisely the same starting position for both scan passes or the data would not correlate.
In view of the foregoing, it would be desirable to develop a method for more completely inspecting the walls of gas pipes under operating flow conditions whereby the phenomenon of "drop-outs" is minimized or eliminated. With this capability, natural gas utilities can monitor the rate of degradation of gas pipes and plan repairs and/or replacements before failures occur.